o Spin quantum numbers or m_s can only take 3 values, i.e. + 1/2and – 1/2which are the 2 spin states ( more than two spin states cannot be held in the orbital). And for an orbital to hold two electrons each of the 2 electrons should be of same quantum numbers i.e. m_s and opposite spins thus, cancelling each other.

o In the case of multi-electron atoms (O), Z_eff is the effective nuclear charge which is the net positive charge of the nucleus, faced by the outer shell (valence) electrons.

• The value of this Z_eff depends on the shielding or screening effect of certain orbital electrons.

• Due to the electrons in the inner shells (nearest to the core), the electron in the outer shell is unable to experience the full positive charge of the nucleus (Z_e).

• Hence, the concept effective nuclear charge is introduced which will also be lowered due to the partial screening or shielding of the positive charge on the nucleus by the inner shell electrons.

Hence the other 3 options -

(i) Z_eff for an electron in a 2s orbital is the same as Z_eff for an electron in a 2p orbital.

(cannot be because 2s and 2p orbital electrons have different screening effect and Z_eff of 2s orbital is > than 2p orbital as 2s is closer to the nucleus.)

(ii)An electron in the 2s orbital has the same energy as an electron in the 2p orbital. (in case of oxygen atom this is not possible as energy of 2p orbital is > 2s orbital.

(iii) Z_eff for an electron in 1s orbital is the same as Z_eff for an electron in a 2s orbital. (also due to the screening effect Z_eff of 1s with no screening is > than 2s ) are all incorrect.